Combined-cycle power generation equipment is manufactured by GE in two basic configurations, single-shaft and multi-shaft. Single-shaft combined cycle systems consist of one gas turbine, one steam turbine, one generator and one heat recovery steam generator (HRSG), with the gas turbine and steam turbine coupled to a single generator in a tandem arrangement. Multi-shaft combined-cycle systems have one or more gas turbine generators and HRSG's that supply steam through a common header to a separate single steam turbine generator unit. Both configurations perform their specific functions adequately, but the single-shaft configuration excels in the base load and midrange power generation applications.
FIG. 1 is a simplified diagram of a conventional combined cycle power plant including a single gas turbine, single steam turbine and generator on a single-shaft, with a reheat, three-pressure-level steam cycle, represented by a heat recovery steam generator (HRSG). The maximum power rating is limited, however, by the maximum capability of a single gas turbine. In combined cycle plants with unfired boilers, approximately two-thirds of the total power output is produced by the gas turbine and one-third by the steam turbine. Supplemental firing of the heat recovery steam generator can increase the total power output and the portion of the total power produced by the steam turbine, but only with a reduction in overall plant thermal efficiency. The largest gas turbines produced today are rated at somewhat less than 350 MW. Therefore, the steam turbine of a single-shaft combined cycle plant has a maximum rating less than 175 MW. This is much smaller than the steam turbines of modem fossil-fueled steam plants of high efficiency, which are more typically 500 to 900 MW. The single-shaft combined-cycle system has emerged as the preferred configuration for single phase applications in which the gas turbine and steam turbine installation and commercial operation are concurrent.
A conventional multi-shaft combined-cycle system configuration, employing a single gas turbine, single steam turbine and a single HRSG, is illustrated in FIG. 2. This configuration may be applied in phased installations in which the gas turbines are installed and operated prior to the steam cycle installation, and it is frequently applied where it is desired to operate the gas turbines independently of the steam system.
The steam turbine in a multi-shaft configuration with a single gas turbine has the same rating as that of the single-shaft plant, but there is no cost or efficiency benefit due to scale. In fact, the multi-shaft arrangement has somewhat higher cost and poorer efficiency, primarily due to the fact that there are two generators, both smaller than the single generator of the single-shaft machine, and there are a larger number of bearings with their associated power losses.
The usual reason for selecting the multi-shaft arrangement is that it permits the operation of the gas turbine in a simple cycle mode, without operating the steam turbine. This might be desirable for peaking operation or for a phased installation in which the gas turbine is installed first and the steam turbine installed at a later date if the need for power increases. In order to operate the multi-shaft plant in a simple cycle mode, means must be provided to dispose of the exhaust heat from the gas turbine. This can be done by dumping the steam produced in the HRSG directly to the condenser, bypassing the steam turbine, or by bypassing the exhaust gas around the HRSG with an auxiliary exhaust stack. Either method adds cost and design and operating complexity to the plant.
In summary, the single-shaft plant of FIG. 1 has the advantages of lower cost, higher efficiency, and simplicity of design and operation; whereas the multi-shaft plant of FIG. 2 has an advantage in operating flexibility in that it can be operated in simple cycle as well as combined cycle mode.
More frequently, multi-shaft plants are designed with two gas turbines and a single steam turbine as shown in FIG. 3. The output of this plant is twice that of those in FIGS. 1 and 2. The output of the single steam turbine is doubled and cost and efficiency benefits due to scale are associated with the steam turbine and condenser. However, to achieve the potential benefits of operating flexibility expected of multi-shaft plants, a separate HRSG for each gas turbine is required, so that the benefits of scale are not achieved by the entire steam plant. Since the output of this plant is doubled, a comparison with a single-shaft plant must be made with one having two identical units of the design shown in FIG. 1. In this comparison, the multi-shaft plant has advantages in terms of cost and efficiency because of the single, larger steam turbine and condenser. However, these advantages are offset somewhat by the following:
1. There are three separate machines rather than two, with a greater total number of thrust and journal bearings with associated power losses. PA1 2. There are cost and efficiency penalties associated with three one-third size rather than two half-size generators. PA1 3. The practical construction of a plant with three separate machines to achieve a degree of independent operation involves considerable complexity in piping, valves and control equipment. The addition of isolation valves in all main steam, and cold and hot reheat steam lines, and in numerous auxiliary steam lines adds cost and parasitic pressure drop and also reduces reliability.
The net effect is that the multi-shaft plant with two gas turbines has a net efficiency advantage over the two half-size single shaft units. Neither configuration is considered to have a clear cost advantage over the other.
With this background, a brief summary of the problems addressed by this invention is provided below. The steam cycle portion of a combined cycle power plant is more expensive to build and has a lower thermal efficiency than is the case for a modem conventional steam power plant. This is due, in part, to basic differences in the thermodynamic cycles of heat recovery and conventional, fired-boiler applications, which can not be changed. However, it is also due to the small size and power rating of the equipment.
Conventional steam power plants benefit in both lower cost and higher efficiency through the economies of scale of large ratings. A traditional rule of thumb regarding cost is that the doubling of plant rating results in a ten percent reduction in cost. The cost of one large generating unit according to this rule would be expected to cost on the order of ten percent less than that for a plant with two half-size units.
Efficiency is also improved with increased size and power rating. As with all turbomachinery, the internal efficiency of the steam turbine is a strong function of inlet volumetric flow, which is directly proportional to rating. Also, as is well known, the thermal efficiency of the Rankine cycle increases with the pressure at which steam is generated. Increasing pressure, however, reduces the volumetric flow of the steam at the turbine inlet, reducing the internal expansion efficiency. This offsetting effect in overall efficiency, however, is much greater at low volumetric flow than at higher volumetric flow. Therefore, an additional performance-related benefit of increasing turbine size is that higher steam throttle pressure can be utilized more effectively.